[S-IT16] Deep Earth Sciences

[SIT16-P07] Lattice preferred orientation of FeO polycrystals developed by large strain under lower mantle pressures

*Bunrin Natsui¹, Shintaro Azuma¹, Keishi Okazaki², Kentaro Uesugi³, Masahiro Yasutake³, Ryuichi Nomura⁴ (1.Tokyo Institute of Technology, 2.Hiroshima University, 3.JASRI, 4.Kyoto University)

Keywords:Rheology, Deformation experiment, Lower mantle, FeO

Seismic tomography has shown that Large Low Shear Velocity Provinces (LLSVPs) with seismic anisotropy extend from the mid-lower to the lowest mantle beneath the African and Pacific. However, its physical and chemical background is mostly unknown. Seismic anisotropy possibly infers the development of crystallographic preferred orientation (CPO), and thus it is important to investigate the relationship between deformation and CPO development in constituent minerals of LLSVPs. However, few studies have discussed the relationship from experimental approaches due to the difficulty of deformation experiments under lower mantle pressure conditions. In addition, whether LLSVPs is thermal or compositional feature is still unclear. Previous studies suggest that the composition of LLSVPs must be 10% denser than that of the surrounding mantle in order to withstand stirring by mantle convection. Therefore, FeO and Fe-rich bridgmanite and periclase are candidate constituent minerals for LLSVPs. In this study, we performed large strain deformation experiments on FeO polycrystals under lower mantle pressure conditions. The aim of this study is to clarify the relationship between the dominant slip system associated with the deformation of LLSVPs and its seismic anisotropy. Our deformation experiments were performed using a newly developed rotational diamond anvil cell under the high-pressure of 8-63 GPa, temperature of 300-600 K, and high-strain of more than 100%. Pt markers for pre- and post-experimental strain measurements were placed in the sample parallel to the sample rotation axis by focused ion beam (FIB) deposition. All deformation experiments were conducted at BL47XU, SPring-8. Strain of deformed samples were determined from the reconstructed cross-sectional images of Pt markers obtained using X-ray laminography imaging techniques. X-ray laminography (12keV) was conducted before and after deformation experiments. In situ X-ray diffraction (XRD) measurements (36 keV) were taken during deformation experiments. Stress and CPO patterns of FeO during deformation experiments were observed by in-situ XRD measurements. The CPO pattern of FeO with B1 structure shows that the 100 axis is strongly aligned along the compression direction and the 111 axis is subparallel to the shear direction after deformation. The CPO of the recovered samples was also determined from Electron backscatter diffraction (EBSD) measurements and compared with XRD results. In our presentation, we discuss the relationship between the experimental results of CPO in FeO polycrystals and the seismic anisotropy of the LLSVPs.